I've been wondering if methyl salicylate (oil of wintergreen) would do
a decent job. Usually, it's used to restore plasticity to dried-out
rubber (one common treatment for hard typewriter platens is "Rubber
Renu", which is MS in a solution of xylol).
Nice fresh minty smell. Good for your arthritis. I haven't tried it
yet as a goo solvent.
--Chuck